The invention relates to a method for measuring, monitoring, and/or controlling directed product movements in fluidized bed and spouted bed systems, the use of suitable measuring devices for this purpose, and fluidized bed and spouted bed systems provided with such measuring devices (fluidized bed systems or spouted bed systems).
A widely used application in the pharmaceutical industry, also used in the food, feed, and fine-chemicals industry, relates to the coating of solid particles in the fluidized bed or the spouted bed using suspensions, solutions, powders, or melts. Here, a certain amount of these particles are set in motion by a stream of processing gas in a fluidized bed or spouted bed arrangement (in the following also called processing arrangement) and fluidized or entrained with the gas flow, with the gas used preferably being air, however perhaps also nitrogen or other suitable gases or gaseous mixtures. The processing chamber is limited at the bottom, inside the processing arrangement, by one or more influx floors, which are embodied as gas distributors and allow distributing the processing gas flow evenly and/or divided into different zones. Such influx floors (for example embodied as sieve trays) prevent by their construction that any particles fall through into the influx area of the processing gas. In the area of this influx floor one or more spray nozzles are localized, by which the particles are sprayed with the spray medium and granulated or preferably coated, generally called “bottom-spray” processes. Such spray nozzles are commercially available in different embodiments and are used particularly as single and/or preferably two or three-component nozzles. In order to yield particularly even coatings on the particles, the particles are sprayed in a direct current and almost entirely dry during the fluidization and/or hovering phase. In order to separate particles, fluidized by the spray nozzle, from the falling, drying particles certain installations and/or guidance devices are installed in the processing chamber. Such installations are described, for example, by Dale E. Wurster et. al. (U.S. Pat. No. 3,196,827 and U.S. Pat. No. 3,241,520). The quality of the product can be further improved by additional installations in the processing chamber. For this purpose, EP 0570546 describes a way to shield the spray nozzle in order to bring into contact the particles to be sprayed only in the area of a well developed spray jet. By this improvement, additionally a higher spray rate can be achieved. For heavier and particularly sensitive particles, for example tablets, installations have been developed according to EP 1 232 003 to protect the product. This way, the product flow is better controlled and the product is protected from wear and mechanical stress. Several appropriate installations allow multi-chamber or continuous systems, such as e.g., shown in U.S. Pat. No. 3,241,520. All above-mentioned installations operate according to the so-called “Wurster principle”.
Even in spouted bed systems operating according to the “bottom-spray” principle (which may also be embodied single or multi-leveled) appropriate product flows and installations can be found, cf. DE 103 22 062 A1 and EP 1 325 775. Here, too, spraying via nozzles occurs in the direction of the primary product flow, which is directed against gravity. The direct current method and/or the direct current principle can be implemented according to the Wurster principle and also according to the spouted bed principles.
In all the processing systems operating according to the Wurster principle today, usually the so-called Wurster pipes are used, standpipes preferably cylindrical in their cross-section, with their distance from the influx floor being variable and which can be ideally adjusted from the outside. The advantage of this coating process according to the direct current principle is a particularly even and homogenous application of the coating material onto the particles provided.
Alternative or in addition to the spray nozzle arrangement according to the bottom-spray method, the spray nozzles may also be mounted laterally at the processing chamber of the fluidized bed or spouted bed system, or also be mounted to certain installations in the fluidized bed or spouted bed system, where here they can spray approximately perpendicular in reference to the product flow and/or preferably here also in the direction thereof. Appropriate conditions are found in spouted bed systems.
The spray rate by which the coating material is applied onto the particles to be coated can either he kept constant over the entire processing progression or adjusted during the progression of the process as well. In this context it is important that the particles are distributed as evenly as possible and are guided through the spray jet at a rate as constant as possible. When in a predetermined, constant spray rate more particles are guided through the spray jet many particles are not or insufficiently sprayed when passing through the spray jet and may be damaged by abrasion or other mechanical stress. This may result in particle breakage and flaking of already applied layers of coating. When at a given spray rate less particles are guided through the spray jet, excess spray droplets cannot be caught by particles. The droplets dry in the processing gas flow and precipitate in the form of fine dust. Additionally, the excess droplets can adhere at the edge regions of the standpipe and form coatings, here. Such coatings hinder an even fluidization and/or formation of spouted beds (more generalized: an even product flow) and can lead to a bad coating result. Fine dust, in turn, can interfere with the formation of a smooth, even surface of the coated particles. For a reproducible and even coating process it is therefore an important condition to ensure a constant product flow (for example in a standpipe of a fluidized bed systems according to the Wurster principle). When the product flow stops by clogging in the product area or in the area of the spray nozzle the quality of the product can be seriously compromised. Particularly when bigger processing arrangements are used having several standpipes or corresponding installations clogging or irregularities are frequently not recognized early enough to allow an intervention.
Using suitable adjustments of the processing conditions and processing parameters, the bottom-spray method (or suitable methods with lateral nozzles) can also be used for the granulation of particles. Here, similar difficulties are found as in the previous paragraph. Both here and there an appropriately good quality of the fluidization must be ensured in order to avoid these problems and processing risks.
In granulation and coating processes the term particle includes all individual materials or objects that can be fluidized in a fluidized bed or a spouted bed systems (preferred variants are defined in the following), which can be granulated in fluidized bed systems or can be coated.
In prior art the quality of the fluidization in the fluidized bed or the spouted bed can be assessed by view panels mounted in the coating chamber, by camera systems, or by the measuring of the pressure difference at the influx floor (e.g., a sieve tray).
View panels are disadvantageous in that they allow an observation of the movement of the particles flowing back from the outside only. The observation by view panels is only possible by using strong light sources, which may result in thermal stress on the product particles, depending on the product. An observation by camera systems requires, due to the high particle speed, a sufficiently quick camera system to render the movements of the fluidized particles distinguishable. Similar to view panels, camera systems need a light source. Camera lenses can be contaminated and clouded by dusting. The view is then only possible to a limited extent. Only expensive rinsing systems with rinsing gas, such as pressurized air, can reduce the formation of coatings. All of the above-described optic ways of control allow only an unsatisfactory qualitative detection of the fluidization behavior (in which particularly the level of fluidization, the concentration of the material flow, and the speed of the material flow are included) of particles in the standpipe. A quantitative statement concerning the fluidization behavior is not possible in this way.
The measuring of the pressure difference is largely dependent on the air distribution and the flow resistance of the influx floor. Particularly in coating processes with more than one standpipe the measuring of the pressure difference only allows statements to a limited extent.
In order to recognize the above-mentioned problems and processing risks and to monitor as well as intervene in a controlling fashion, if necessary, the direct measurement of the product flow in the standpipe is required. A direct measurement is possible by capacitive measuring methods or by measuring the electric resistance in the standpipe. These methods are largely influenced by product humidity or material characteristics, though. Frequently, the di-electricity constant of the product also changes during the coating process such that the capacitative measurements occur under changing conditions. Additionally, the difference of the capacity changes in a filled standpipe is very little compared to an unfilled standpipe, even under ideal conditions. A reliable measurement or assessment of the signals is impossible. Interfering signals, for example by the influence of measurement lines, limit the application of this method even further. In the measurement of the electric resistance dusting or contamination occurring during the process can lead to faulty measurements at the measuring electrodes. The resistance measurement can also be interfered with by purified and deionized water, e.g., used in the production of pharmaceuticals.
From WO 98/44341 A1 a method is known to monitor and/or control and regulate a granulation, agglomeration, instantization, coating, and dehydration process in a fluidized bed or a mobilized pouring by determining the product moisture as well as the ventilation apparatus for performing such methods. The dampening of high-frequency waves of less than 100 MHz or microwaves through moisture present in a fluidized bed is determined by a sensor (embodied as a planar sensor), installed in the external wall of an appropriate fluidized bed system and ending appropriately flush to its interior side, being the moisture sensor. The measuring signal is essentially described depending only on the moisture level and the product temperature. The relevant resonance frequency is described for the entire fluidized bed, not in reference to any individual particles. Electronic measuring signals and product moisture measured are correlated for calibration “off-line”.
In WO 98/44341 A1 the overall product moisture level is measured—the fluidization must still be determined by other methods, such as the above-mentioned ones, with all their potential shortcomings.
In DE 195 045 44, the charging with and the speed of carbon dust through pipes for controlling the firing in a boiler is described (primarily via adjusting the amount of supply air for the combustion) in a coal-fired power plant, using microwaves. The ability to use microwave technology for conditions of variable moisture content during spraying is not obvious or described. Here, the purpose is the regulation of a downstream arranged combustion, not the regulation within the chamber of the microwave measuring.